In southern California's Antelope Valley, 24,000 silver-bright mirrors have been positioned to reflect light on two 50-meter-tall towers. And at 11:08 A.M. local time Wednesday, this concentrated light heated steam in those towers to turn a turbine—the first "power towers" in the U.S. to convert the sun's heat into electricity for commercial use.

Dubbed Sierra SunTower, the power plant can produce five megawatts, enough to power roughly 4,000 local homes at full capacity—and provide the modular blueprint for larger plants in California and New Mexico, according to eSolar, the Pasadena start-up behind the power plant.

"We call this a commercial demonstration," says eSolar senior vice president of engineering Craig Tyner. "A 46-megawatt commercial design will incorporate 16 of these towers, two of which we have at Sierra" as well as more than 200,000 mirrors capable of generating at least 90 gigawatt-hours of electricity per year.

Harvesting the sun's energy as heat is hardly a new idea: During the energy crisis of the 1970s, designs for solar thermal power plants took off. And ever since 1984, vast arrays of curved mirrors have been concentrating the sun's rays on pipelines filled with synthetic oil at the Solar Energy Generating Systems (SEGS) power plant in California's Mojave Desert. But power towers, at least theoretically, would be more efficient than these so-called parabolic trough designs, because all the heating and fluid is contained in one central tower.

The key to eSolar's design are the mirrors—known as heliostats in the concentrating–solar power industry. By precisely calibrating the mirrors with computer algorithms driving shoe box–size motors, eSolar can build its sunlight-harvesting power plants with many more small, flat mirrors, roughly one square meter in size, as opposed to the large, curved specialty mirrors employed in other designs. "We're using more software algorithms and less steel," says Bill Gross, CEO of the Google-backed solar company who, at the age of 15 in 1973, started Solar Devices, a firm which sold plans and kits for solar power, before pioneering pay-to-click advertising for search engines in the 1990s.

But the multiplicity of mirrors could also prove the technology's weakest link. "The question is going to be the maintenance of all those heliostats," says Mark Mehos, program manager for concentrating solar power at the U.S. Department of Energy's National Renewable Energy Laboratory in Golden, Colo. "You have orders of magnitude more heliostats that you're going to have to maintain and that you're going to have to track."

Sierra SunTower has a number of environmental points in its favor in addition to the renewable energy it generates. It employs reclaimed water for its cooling and was built near existing transmission lines on what used to be farmland rather than the pristine desert areas nearby that have provoked opposition to some other planned solar power plants. "This model makes the destruction of public lands unjustified," says David Myers, executive director of nonprofit environmental group the Wildlands Conservancy. "There are 200,000 acres in southern California of lands that are degraded or disturbed that we support for solar electricity."

Gross, for his part, estimates that the entire state of California's peak electricity demand could be generated from an eSolar field of 65 square kilometers. That electricity would cost no more than 13 cents per kilowatt-hour, the current average price of power in the state, although the company refused to reveal how much the existing plant cost to build or what price Southern California Edison is paying for the power. "This is a second industrial revolution to power the planet cost-effectively from the sun," Gross says.

Of course, the sun doesn't always shine and, at present, the eSolar design has limited capacity to store energy—either as heat or electricity—nor does it supplement production by burning natural gas as some other existing concentrating solar power plants do. "It's 100 percent solar," Tyner says, although he anticipates incorporating heat storage in molten salts or some other energy storage technology in the next generation design due in two years or so.

As it stands, eSolar expects to generate five megawatts of electricity with its refurbished 1940s-vintage General Electric steam turbines roughly 25 percent of the time, which coincides generally with peak demand for electricity, Tyner says. And the efficiency with which a field generates electricity can be improved in the future by moving to higher temperature operation, he adds; the current design employs steam at 440 degrees Celsius and 63 kilograms per square centimeter of pressure at peak capacity.

The company has paired with NRG Energy, Inc., to develop two full-scale power plants totalling 92 megawatts in New Mexico as well as 337 megawatts of further power towers in California. The company plans to begin construction of the New Mexico plant next spring and complete it by the summer of 2011. It has also partnered with the ACME Group in India to develop roughly one gigawatt of power tower capacity in that country. And eSolar is not alone. Rival developer BrightSource has contracted with Southern California Edison for 1,300 megawatts worth of similar power towers, among other projects gaining steam.

Whether this is truly the dawn of a new, cheap solar energy era will be proved at power towers like this one—as well as two operated by Abengoa in Spain. After all, "the one hurdle towers still face is demonstrated performance and reliability. This plant should help that," NREL's Mehos adds. "It's a big step forward for just demonstrating this technology. Every mirror counts."

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